The present invention relates to a pressure sensor comprising a diaphragm which is joined around the periphery to a substrate so as to form a chamber, and whose surface facing away from the substrate is exposed to a medium whose pressure is to be measured.
In pressure sensors of this kind, the diaphragm is preferably made from a low-cost spring material suitable for economical mass production, such as ceramic, glass, quartz, single-crystal material, or base metals. A material especially suited for the diaphragm is oxide ceramic, particularly alumina ceramic. The diaphragm materials used must meet very different requirements which relate to the following aspects in particular:
1. the desired elastic qualities, such as creep strength, hysteresis-free operation, etc.;
2. the technique used to join the diaphragm to the substrate, such as soldering, welding, fusion by glass frit, or the like;
3. the resistance to influences of the medium whose pressure is to be measured, particularly corrosion and abrasion resistance.
There is practically no diaphragm material which meets all these requirements. Depending on the predominant requirements in the various applications, pressure sensors with different diaphragm materials must therefore be made available, which adds to manufacturing and warehousing costs. With respect to the other requirements, trade-offs usually have to be made.
It is the object of the invention to provide a pressure sensor of the above kind which, having a diaphragm made from any of the conventional low-cost spring materials, which can be selected with regard to the desired elastic qualities and the joining technique used, can be mass-produced in an economical manner, the diaphragm have a high resistance to influences of the medium whose pressure is to be measured, particularly to corrosion and abrasion.
To accomplish this, according to the invention, a layer of silicon carbide is applied to the surface of the diaphragm facing away from the substrate.
The silicon-carbide layer applied to the diaphragm in accordance with the invention is highly resistant to both corrosion and abrasion. It thus acts as an anticorrosive and antiabrasion layer, which prevents any chemically or mechanically aggressive medium from coming into contact with the diaphragm material proper. The diaphragm itself can therefore be made from a conventional low-cost material which can be selected with regard to other requirements, such as elastic qualities and joining technique used.
For the anticorrosive and antiabrasion layer of silicon carbide, a thickness of about 1 to 10 .mu.m is sufficient. Silicon-carbide layers of such a thickness can be formed quickly, at low cost, and with good reproducibility by coating the surface of the diaphragm with silicon carbide by chemical vapor deposition (CVD). This method has been known for some time and is especially suited for economical mass production. Since the deposition of silicon carbide by the CVD process takes place at temperatures of about 1000.degree. C., this method is only suitable for coating diaphragm materials that can withstand this temperature, such as ceramics, quartz, single-crystal materials, and certain metals. Plasma enhanced chemical vapor deposition (PECVD), a method which became available only recently, makes it possible to deposit silicon carbide at much lower temperatures, namely about 100.degree. to 200.degree. C., so that it is also suitable for coating less heat-resistant materials, such as glass. If PECVD is used, the diaphragm may be coated after the parts have been assembled.
In any case, chemical vapor deposition results in silicon-carbide layers of high elasticity and low flexural rigidity which adhere well to all diaphragm materials and have no or only very low residual stresses. Reactions of silicon-carbide layer on the diaphragm are therefore negligible.
An essential advantage of the application of the silicon-carbide layer by chemical vapor deposition consists in the fact that a very dense coating is achieved on both smooth and very rough surfaces, so that the silicon-carbide layer is free of microcracks and imperfections. Thus, even the rough surfaces of ceramic diaphragms can be coated without first having to be polished.
As silicon carbide is a semiconductor, the silicon-carbide layer deposited on the diaphragm can be made electrically conductive by suitable doping. It can then additionally be used as an electrostatic shield.
Further advantageous aspects and developments of the invention are defined in the subclaims.